Download genetic equilibrium

Genes and Variation
• It’s an organism’s phenotype (traits)
that make it better fit its
environment
• And these traits come from the alleles
it has
• All of the alleles in a population is
known as its gene pool
• Evolution is a change in the gene
pool of a population over time.
• This involves changes in allele
frequency over time.
Sources of Variation
• Random mutation…alleles can
randomly change…we have
approx 300 mutations from our
parents, but most are
unnoticeable.
• Recombination…independent
assortment and crossing over
during meiosis make our genes
different from our parents
Sources of Variation
• Lateral gene transfer…genes
transferred from 1 organism to
another
– e.g. bacteria often exchange DNA
– not so much in higher organisms, like
humans and most animals and plants
Phenotypes
• So it’s the phenotypes that we really
see in evolution
• Traits can be caused by 1 gene or
many. Most are polygenic …
controlled by many genes
• So you don’t get just 2 phenotypes,
like what we studied in genetic unit
• Get a range of phenotypes, like we
saw with bar graphs for height and
hand size in human adaptation lab.
Bell Curves
• With polygenic
traits, you
get this
range of
phenotypes
rather than
just 2
• If this curve changes, evolution has
occurred
3 kinds of Natural Selection
• Directional selection
– Individuals at one end of curve are most
fit for environment
– e.g. birds with bigger beaks get more food
3 kinds of Natural Selection
• Stabilizing selection
– Organisms with average trait survive best
– e.g. babies with medium weights, too
heavy or too light = health problems
3 kinds of Natural Selection
• Disruptive selection
– Organism at both extremes are fittest
– e.g. medium sized seeds less common,
large and small beaks are advantageous
Genetic Drift
• So that’s how evolution occurs by
Natural Selection
• Can also occur by accident or chance
and not by natural selection
• Evolution by random chance is
called genetic drift
• Usually happens in smaller
populations, because larger ones
can recover from these accidents.
Genetic Drift
• Bottleneck effect
– When an accident
(e.g. disease,
forest fire,
hurricane) wipes
out a large part of
the population
– the allele
frequency may
change, but not
because of
natural selection
Genetic Drift
• Founder effect
– When a few individuals leave the group and
start a new population
– They may not have all the alleles in the original
population, so the allele frequency has
changed, but not because of natural selection
Genetic Equilibrium
• When a population isn’t evolving,
it is in genetic equilibrium
• Allele frequencies remain
constant according to the
Hardy-Weinberg Principle:
p+q=1
p2 + 2pq + q2 = 1
p = frequency of dominant allele
q= frequency of recessive allele
Genetic Equilibrium
• Hardy-Weinberg Principle predicts
populations will stay in genetic
equilibrium unless:
– There is nonrandom mating (i.e. artificial
or sexual selection, rather than natural
selection)
– Small population size (genetic drift has
big effects, then)
– Gene flow (alleles enter or leave the
population through immigration (coming
in) or emigration (leaving)
Genetic Equilibrium
• Hardy-Weinberg Principle predicts
populations will stay in genetic
equilibrium unless:
– Mutations happen to change the alleles
available
– Natural selection happens where one
genotype becomes more fit for the
environment, and survives to reproduce
more often
• H-W principle is kind of a “what if”. We
know what it takes for evolution to
occur, what would it take for evolution
NOT to occur?
Speciation
• A species is a population who
members can interbreed and
produce fertile offspring.
• Evolution has occurred when 1
species is so different from
another that they can’t mate
any more.
• This is called reproductive
isolation
Kinds of Reproductive Isolation
• Behavioral isolation
– When organisms act differently and stop
mating, e.g. birds in same area use
different songs/rituals to attract mates
• Temporal isolation
– When organisms in the same area mate
at different times from each other, e.g.
orchids in rainforest blooming at
different times, frogs in wetlands mating
at different times
Besides Reproductive Isolation
• Geographic Isolation
– When a population is separated by a
river, lake canyon, mountain and stop
mating, e.g. Darwin’s finches on
different islands, plant seeds that get
carried to new areas
Example: Darwin’s Finches
• Birds arrive from
mainland onto
islands (founder
effect)
• Birds travel to
another island
(geographic
isolation)
Example: Darwin’s Finches
• Changes in gene pool
because of natural
selection e.g.
different seeds to
eat = changes in
beak size/shape;
different plants =
different colored
feathers to
hide/stand out
Example: Darwin’s Finches
• Behavioral isolation:
birds fly back, but
are different now,
don’t mate
• Birds continue to
compete, move
around and evolve
Molecular clock
• Very simply, by looking at how
many differences there are in
DNA, a biologist can tell how
long ago the species evolved
from each other.
Rates of Evolution
• Gradualism: very
slowly with changes
barely noticed
• Punctuated
equilibrium: short
bursts followed by
long time frames
with no change
Kinds of Evolution
• Divergent evolution
– What we’ve been talking about all along
– One common ancestor leads to many
descendants
– Leads to homologous and vestigial
structures
Kinds of Evolution
• Adaptive
radiation
– Evolve to inhabit
all habitats and
niches
– e.g. dinosaurs…live
on land, in water,
in air, in swamps,
herbivores,
carnivores, etc.
Kinds of Evolution
• Convergent evolution
– Organisms from different ancestors
evolve to have similar traits
– leads to analogous structures
– e.g. grassland birds Darwin observed:
ostrich, emu, rhea
Kinds of Evolution
• Coevolution
– When organisms rely on each other so
much that as one changes, the other
does also
– e.g. plants and the birds/ insects that
pollinate them
Extinction
• If evolution doesn’t happen,
species can go extinct:
– Very slowly, a few species at a time =
background extinction.
– Relatively quickly, many species at once =
mass extinction
• 5 mass extinctions in Earth’s history
– Last one was 66 mya…dinosaurs
– Biggest one was Permian...250mya…96%
of all species wiped out!
Convergent
Evolution
Divergent Evolution
Divergent
Evolution
Adaptive
Radiation
• Evolve to fill
many niches,
e.g. live in
many places
or eat many
different
things
Adaptive Radiation